1. Field of the Invention
The present invention relates to an automa,tic telephone answering system of the type which uses dual-tone, multiple frequency signals (DTMF) as remote control signals and, more particularly, to a recorder control apparatus used in such an automatic telephone answering system.
2. Background Discussion
A conventional automatic telephone answering system which can be remotely controlled by DTMF signals comprises, as shown in FIG. 10, a recorder 91 connected to a transformer 94 which is in turn connected to a telephone line, a remote control signal receiver/decoder section 93 connected to the transformer 94 and consisting of a DTMF receiver 95 for receiving DTMF remote control signals and microcomputer 96, and a line DTMF discriminator means 97 implemented by the micro computer 96 for discriminating DTMF signals from the telephone line. The recorder 91 records messages from the telephone line in a record mode and plays recorded messages and transmits them to the telephone line in a play mode.
It has heretofore been known that the recorder 91 records in the record mode not only the message of a caller but also the DTMF signals generated when the caller touches the telephone keys. The DTMF signals which are thus unintentionally recorded and generated upon playback should not be interpreted as true remote control signals. If these "false"DTMF signals are played from the recorder in the play mode, these signals are transmitted to the telephone line and applied to the remote control signal receiver/decoder sections 93 as well. In some cases, the receiver/decoder section 93 operated in the play mode accordingly mistakes the false DTMF signals for true DTMF remote control signals coming from the telephone line and this causes erroneous operation of the automatic telephone answering system.
In order to prevent such erroneous operation, the conventional automatic telephone answering system is provided with a line DTMF discriminator means. This discriminator means responds to a detected signal (DS) which is qenerated by the DTMF receiver of the receiver/decoder section 93 to indicate detection of a DTMF signal and, when the DS signal becomes high in level to indicate detection of such a signal, the discriminator means switches the line playing/transmitting circuit 99 off to detect whether or not the DS signal becomes low in level. That is, in the case where the DS signal becomes low when the playing/transmitting circuit 99 is switched off, the discriminator means 97 determines that the received DTMF signal is a false DTMF signal played by the recorder 91 and prevents the system from responding to this false DTMF signal. On the other hand, in the case where the DS signal remains high, the discriminator means 97 determines that the received DTMF signal is a remote control DTMF signal from the telephone line and causes the remote control signal decoder section 93 to respond to it.
In the conventional automatic telephone answering system described above, discrimination of the veracity of the remote control DTMF signals takes a relatively long time period, that is at least 80 ms (preferably 100 ms when variation in the quality of ICs is considered), since it is performed by checking the DS signal of the DTMF receiver (such as receiver 95).
More specifically, in order for the DS signal to become definitely high (i.e. recognition of DTMF signal), it takes at least 40 ms from the beginning of the DTMF signal and, on the other hand, in order for the DS signal to become definitely low, it takes at least 40 ms from the end of the DTMF signal. In the conventional system described above, therefore, a time period of at least 80 ms is necessary from the beqinning of the DTMF signal to completion of the discrimination of the veracity of the signal by the discriminator means 97.
As a result, in a case where one attempts to make the remote control from a telephone set which generates DTMF signals of durations shorter than 80 ms or where DTMF signals of such durations are used as remote control signals, a problem encountered in the conventional system described above has been that these DTMF remote control signals are all invalidated. Generally, duration of normal DTMF signals varies qreatly depending on the type of telephone set used. For example, some types of telephone set have a signal duration of as short as 50 ms. Thus, a problem arises in using this type of set to generate DTMF signals to remotely control the machine.
Further, in order to accurately discriminate the remote control DTMF signals in the conventional system, difficult design problems are encountered with respect to the setting of the delay times and guard times for detection of DS signals.
Accordingly, an object of the present invention is to provide an automatic telephone answering system which is simple in construction and remotely controllable without the need for validation of the remote control DTMF signals.
Another object of the present invention is to provide a recorder control apparatus for an automatic telephone answering system which is capable of responding to remote control DTMF signals of shorter duration.